The term "third-degree" is a compound formed from the ordinal number "third" and the noun "degree," meaning a step or grade. Its literal sense, denoting the most severe category of burns, originates from a late 19th-century medical classification system where burns were ranked in ascending severity by degrees. The figurative and now more common meaning, referring to intense and harsh interrogation, emerged in early 20th-century American police slang, logically extending the concept of ascending intensity to the metaphorical "steps" or "levels" of pressure applied to a suspect. Thus, the term's evolution perfectly illustrates how a concrete, medical ranking system was borrowed to describe an abstract social process of escalating severity.

Structure: third + degree - third: From Old English þridda, from Proto-Germanic þridjô, meaning "coming next after the second." Functions as a numerical ordinal. - degree: From Old French degre, from Vulgar Latin degradus, from Latin de- ("down") + gradus ("step"). Functions as a noun meaning a step, stage, or rank.